Event handler priorities

While logging in, the `LastLoginListener` tries to update the `lastLogin` using the `DelegatingUserManager`. However, the `$currentUser` in the `UserDiscriminator` isn't set - the event which sets this is scheduled to run after. 

I've given the subscribed events in `AuthenticationListener` the maximum possible priority so that they'll run before other events which might be trying to use the `UserDiscriminator`. 

I don't know whether there'd be a case where other subscribers would need to be called before these. If there might be, the priority of these events may need reducing.